1. Field of the Invention
The present invention pertains to a method providing substantially improved irradiation of large areas of three-dimensional (3D) objects. More particularly, the present invention pertains to a method of projecting a high level of light flux in a non-uniform pattern onto a three-dimensional target surface so as to more uniformly treat the surface.
2. Description of the Prior Art
Various manufacturing processes include treating 3D surfaces by illuminating surfaces with radiant energy, for example, ultraviolet light or irradiation of surfaces by particle beams, for example, electron beams. The radiation treatment may be related to curing, polymerization, oxidation, purification, disinfections, or some other procedure. By way of example, the manufacture of wood components for furniture involves the application of a clear coating onto the surface of the component for the purpose of surface protection and improvement of the appearance of the wood surface. The clear coats are resins or polymer-based materials that are applied as liquids and require heating or other processing to become solids. The curing of clear coatings by thermal treatment is not instantaneous and usually requires times ranging from minutes to hours. Non-thermal curing using radiant energy to polymerize the clear coat is rapid in comparison to thermal treatment. This speed is achieved because the energetic beam that causes the needed chemical change the moment it is applied to the surface. Obtaining a high quality, uniform product requires irradiating the surface with a uniform high level of radiation energy flow rate (flux) that meets or exceeds processing parameters over the entire target area. Otherwise, irregularities in the finished product will result.
Existing devices for irradiation or illumination usually produce a high level of irradiance or illuminance within a narrow depth-of-field with respect to the radiation source. This limits existing radiation sources to the treatment of three-dimensional objects that are smaller than the depth-of-field when a uniform irradiation of product is required.
The treatment of 3D objects that are larger than the depth-of-field of the radiation source requires a means of controlling the uniformity of the net spectral irradiance, Iλ. Iλ is related to the net surface spectral radiance Fλ Fλ is defined as the energy outflow rate (radiant light flux) in the space angle dΩ per unit surface area of the source within a small wavelength range λ to λ+dλFλ=δ4E/δAδtδΩδλ(W.m−2.sr−1nm−1)
The net spectral irradiance Iλ is related to the net surface spectral radiance, Fλ, by considering the directional dependence of the energy flow rate (flux) striking the surface. The net spectral irradiance Iλ is defined by:Iλ=δ4E/cos θδAδtδΩδλ(W.m−2sr−1nm−1)wherein Iλ is a subset of energy flow rate (flux) in the wavelength interval (λ,λ+dλ2), flowing toward the targeted 3D surface element δA (with a unit normal n) within a solid angle δΩ. The angle θ is the angle between the normal of the surface element and the direction of the incident light beam. The irradiance is a projection of the radiance onto the surface, and for the case of a light beam from an isotropic point source is a function of the shape of the surface if that surface differs from that of a sphere centered on the source.
Actual light sources are non-isotropic and spatially extended. In general, 3D objects have arbitrary shapes where the direction of the surface normal varies in a complex fashion. To achieve uniformity of the radiance for a 3D object requires either a complex light beam resulting in irradiance that varies in a point-by-point manner over the object, or a complex sequence of motions of the light source, 3D object, or both, the light source and 3D object which averages the irradiance over the angular space of the 3D object or light beam.
Various techniques have been used to control the irradiance from a single radiation source over the surface of 3D objects. The techniques are also known regarding illumination where the same techniques are used to improve the performance of visible lighting systems for a number of purposes.
U.S. Pat. No. 4,839,522 discloses an ultraviolet light treatment system that uses transmission through a translucent material or diffuser to induce multiple scattering that randomizes the direction of light rays. When integrated over any point on the surface this randomization reduces the variation in the energy or power impinging on a surface element. A drawback inherent in this approach is that multiple scattering increases the path length of a ray in the translucent material and its absorption. Such systems may be inefficient given that energy is removed from the treatment beam. The absorption of energy in the diffuser may lead to excessive heating of the optics and a need to provide active or passive cooling devices to prevent damage to the optics or any adjacent component of the treatment system. Finally, energy absorption may damage the optical system in ways unrelated to purely thermal effects.
The treatment beam can also be modified to improve its coupling to 3D surfaces by breaking the beam into smaller sub-beams or beamlets. The beamlets can be directed onto a point on the surface to provide a spread of angles or by superposition to increase the flux. U.S. Pat. No. 6,271,532 is illustrative of the use of a facetted optical element to break a beam into beamlets. With the '532 patent a mirrored surface is dimpled to create an array of facets that have variable focal lengths. This modifies the depth of focus by increasing it. U.S. Pat. No. 6,166,389 employs an approach where a sol-gel generated film is selectively applied to a transparent optical element like a lens to scatter light in a specific direction to modify a beam profile. Depending on the optical element, radiant energy can be lost through absorption or scattering. Modification of optical components to increase beam divergence or randomness can reduce the original performance of the optical component.
3D objects can be made into 2D objects by slicing, sectioning or projection. Japanese Kokei JP-4-173233 shows that projecting a series of appropriate 2D images on a resin can create 3D shapes. In the preferred embodiment, a narrow beam is traced into the resin and draws the desired 2D pattern of the appropriate slice. By fabricating a series of slices a 3D object is created. The same technique can be used to scan a beam over a 3D object or to project a series of patterns that happen to irradiate all parts of a 3D object. A significant limitation of this technique is the requirement that no part of the 3D object blocks the incident beam. If this occurs, then parts of the 3D object will not receive the beneficial treatment provided by the radiant energy.
The limitation of Japanese Kokei JP-4-173233 can be overcome by the approaches embodied in Japanese Kokei JP-5-338042 that teaches the use of multiple lamps or energetic beam sources placed around a 3D object to direct light beams onto every part of the treatment object. The lamps or energetic beam sources can be placed at specific locations to control the irradiance or dose over all parts of the object. The '042 Kokei also reveals the use of arrays of mirrors to split the beam from a single light source into multiple beams that are directed in a controlled manner to accomplish the same function as an array of multiple light or energy beam sources. This approach to 3D curing increases the cost of the curing system by increasing the number of curing beam sources or the number of optical components like lenses, mirrors or optical fibers.
Japanese Kokei JP-8-257468 and U.S. Pat. No. 6,566,660 disclose radiation or energetic beam sources attached to a mechanical device with numerous degrees of freedom to permit the energy sources to be moved in a controlled motion over a 3D object to control the irradiance or energy dose. The energetic source can be held in a fixed position and the treatment object moved in a manner that allows the treatment beam to be scanned over all parts of a 3D object.
Japanese Kokei JP-5-338042 teaches the use of a turntable to move a 3D object during curing to improve the irradiance profile over a 3D object and to ensure that surfaces facing away from an energy source also receive exposure to an energetic beam. While an effective method for improved treatment of 3D objects, there is an increased cost for the mechanical components needed to move the treatment beam source.
The use of holograms to fabricate 3D objects from resins is described in Japanese Kokei JP-6-305032. Lasers are used to irradiate the holographic elements and project the light patterns necessary to treat the resins and make 3D parts. This technique is similar that disclosed in Japanese Kokei JP-4-173233 which teaches the use of a series of two-dimensional projections to construct a 3D part. Kokei JP-6-305032 uses a 3D projection to create a part during an exposure. A drawback in the holographic technique is the requirement to use a coherent radiation source or laser. Lasers are very inefficient in the conversion of electrical energy into radiant energy and have a small aperture and narrow beam. The latter limitation makes it hard to project a laser beam over a large solid angle.